1. Field of the Invention
This invention relates to a method of forming a fine resist pattern on a semiconductor substrate. More specifically, this invention relates to a lithography technique utilizing an electron beam or an X-ray beam in an exposure process for forming a resist layer on the substrate.
2. Description of the Prior Art
Prior art lithography techniques utilize ultraviolet light to expose a resist layer formed on a semiconductor substrate. However, the ultraviolet light technique has resolution limitations and is not suitable for a resist pattern having dimensions of less than one micron. Because of a trend toward higher integration of semiconductor devices, the need to form a smaller resist pattern (less than one micron) has developed. The development of electron beam and X-ray lithography has, to some extent, allowed formation of patterns this small.
In electron beam lithography, an electron beam having an energy level of about 20 to 30 KeV (Kilo-electron-Volts) is applied to a resist layer (having a thickness of about 1 micron) formed on the semiconductor substrate. However, the energy level of the electron beam is of a level sufficient to penetrate the resist layer, and the electrons collide with and penetrate the substrate under the resist layer. The penetrating electrons induce forward and backward scattering of electrons. Therefore, the resist layer is subjected to extra exposure, particularly by the backwardly scattered electrons. Because the scattering pattern of the electrons is isotropic, portions of the resist layer adjacent to the pattern used in forming the resist layer are affected by the scattering electrons, even though the portions are not directly exposed. This partial exposure causes pattern precision and contrast deterioration and/or blurring of the pattern.
FIGS. 1(a) and 1(b) are cross-sections of a semiconductor device in which a negative type resist layer 1 is formed on semiconductor substrate 2 and is exposed to an electron beam having a width W. Hereinafter, this negative type resist electron beam lithography process is abbreviated as a negative EB resist (if a positive-type resist layer is used, it is called a positive EB resist). After resist layer 1 is developed, the pattern 1' is obtained. As shown in FIG. 1(b) , pattern 1' is equal to the electron beam width W at the top surface of the patterned resist layer. However, the width of pattern 1' gradually broadens toward the substrate 2 due to the above-mentioned electron scattering. When a positive EB resist layer 1 is used, the formed resist pattern 1' is reversed, as shown in FIG. 2.
In X-ray lithography, X-rays collide with the substrate after penetrating the resist layer causing the emission of electrons (photons) from the substrate surface. As with electron beam lithography, the resist layer is exposed to the scattering photons, resulting in a decrease of contrast and blurring of the resist pattern. This phenomena occurs particularly when SOR (Synchrotron Orbitary Radiation) is used as the X-ray source, and especially when a very short wavelength (for example, less than one Angstrom) is utilized.
Several prior art methods have been developed to solve this problem. One method is to control the acceleration voltage of the electron beam. When the acceleration voltage for electron beam is increased, the electron penetrates more deeply into the substrate, and the backward scattering electrons from this deeper penetration of the substrate have a smaller energy level, lowering the effect on the resist layer. Therefore, the application of an electron beam of high voltage is preferable to reduce pattern blurring.
Another method is to apply a multilayer resist system, in which plural resist layers are laminated on the substrate. An upper resist layer is an electron beam sensitive layer and a lower resist layer, having a greater thickness than that of the upper layer, is a buffer layer for absorbing scattering electrons.
The above methods are commonly known techniques for both EB and X-ray lithography. Other methods also exist, however, further improvement of resolution to avoid blurring when using EB or X-ray lithography is, of course, desirable.